Stent delivery system with radially stabilized catheter

ABSTRACT

A catheter for a medical apparatus comprising: an inner catheter ( 100 ) having a distal end and a proximal end; and a sheath ( 202 ) of polymer material, having a distal end and a proximal end, disposed around at least a portion of the inner catheter, said sheath being retractable in a proximal direction relative to the inner catheter to perform an actuating step at the distal end of the system, by the application of an endwise tensile stress to the proximal end of the sheath, characterized in that: the inner catheter resists the associated radially-inward contraction of the sheath which arises from the applied tensile stress during said actuating step.

This invention relates to medical devices and, more particularly, butnot exclusively, to devices for performing minimally-invasive surgicalprocedures.

When performing medical operations, it is of paramount importance thatthe surgery itself causes as little trauma to the patient as possible.For this reason, medical science is continually developing new andimproved methods for treating patients and reducing the risk of severeconsequences arising from the procedure itself. In recent years,techniques such as keyhole surgery and endoluminal or transluminaltreatments, which avoid the need for traumatic open surgery, have becomecommon practice in many parts of the world. Benefits to the patientinclude reduced external scarring, minimal trauma during surgery,reduced risk of infection and shorter recovery periods. Correspondingly,there has been a demand for new and improved surgical equipment, capableof performing minimally invasively the functions required tosuccessfully treat the numerous and varying medical conditions. As aresult, there is ever-increasing pressure for individual instruments tocombine an increasing range of functions, to be of smaller dimensionsand to maintain or improve the accuracy and manipulability of thedevices.

In the specific field of transluminal or endoluminal surgery, one driveis towards reducing the diameter of devices to be inserted into andguided along a body lumen, in order to allow surgical procedures to beperformed in narrow conduits such as blood vessels which areinaccessible to larger devices. At the same time, it is necessary toensure that the devices can perform to a high level of accuracy and canbe easily directed and controlled by the surgeon, particularly when theyhave to be guided through the tortuous passageways such as are definedby inter-connecting blood vessels within the human body.

Common procedures include the treatment of coronary diseases anddeficiencies by the endoluminal implantation and deployment of stents orreplacement valve structures at locations where the natural bloodvessels have become defective, blocked or damaged. In such a procedure,a medical device is loaded onto or into a delivery catheter in acompressed or reduced-diameter configuration. The catheter is theninserted through an incision into a blood vessel, typically the femoralartery, and guided through the passageway of inter-connecting bloodvessels to the site requiring treatment. The medical device then expandsor is expanded within the passageway at the treatment site, where itremains. The delivery catheter is then retracted through the passagewayand removed through the same incision.

EP-A-0 836 447 discloses a stent delivery system comprising an innercore, having a proximal end and a distal end, made from a wire coil; astent concentrically arranged around the inner coil near the distal end;a sheath concentrically arranged around the inner coil extending fromthe proximal end to a distal end proximal of the stent; an outer sheathcovering the stent; and means for retracting the outer sheath. When thedelivery system has been inserted and guided to the correct location,the outer sheath is retracted, releasing the stent contained therein.During insertion of the catheter, the wire coil is flexible, allowing itto advance through the tortuous passageway defined by theinter-connecting blood vessels. During retraction of the outer sheath,the wire coil provides sufficient rigidity and resistance to axialcompression to allow retraction of the outer sheath.

EP-A-1 181 906 discloses a similar stent delivery catheter, including awire coil and further including a covering that fits over the coil tohelp resist buckling in bending and compression.

There is associated with endoluminal surgical procedures the need tovisualize the position of the catheter as it is advanced through thebodily lumen, and to ensure that the device is properly located priorto, during and after deployment. In order to visualize the position ofthe advanced distal end of the catheter, pulses of visualizing fluid areinjected into the bodily lumen so that the catheter distal end can beseen using visualisation means, such as radioscopy or fluoroscopy. Inorder to transport the visualizing fluid, prior art devices eitherprovide a separate lumen within the catheter, for transportingvisualizing fluid from the proximal end to the distal end of thecatheter, or visualizing fluid may be injected through a guide catheter,within which the delivery catheter is advanced. In the latter system,the catheter-can be provided with a shaft that is narrow except at thedistal end, where the medical device is held, so that a sufficientvolume of visualizing fluid can flow within the guide catheter lumenaround the shaft and is not restricted until it reaches the distal end.A continuing preoccupation for catheter designs is how to providesufficient quantity of visualizing fluid from a proximal end to a distalend of the delivery catheter without requiring an increased-diametercatheter and without restricting the flow past the delivery catheter.

With the prior art devices, there is often a trade-off between theflexibility of the delivery catheter and the resistance of the deliverycatheter to compressive forces during retraction of the sheathcontaining the medical device. One method of mitigating this difficultyhas been to reduce the length of the sheath relative to that of themedical device, and thereby reduce the friction force resistingretraction of the sheath during deployment of the medical device. A pullwire within the catheter can be used for controlled retraction of thesheath once the catheter is correctly positioned. In some prior artdevices, the outer sheath is braided or has wire reinforcement toprevent the sheath from lengthwise stretching, so that the sheath willretract when the actuating means is operated and not just becomelongitudinally stretched. This results in the sheath having increasedthickness. Therefore, another problem is to provide a system withreduced overall diameter that is flexible and controllable, withoutlosing compression-resistance in the inner coil, and without the sheathstretching during retraction and incorrectly deploying or failing todeploy the medical device.

In accordance with one aspect of the invention there is provided apercutaneous, transluminal system with a distal end for inserting into abody and a proximal end to remain outside the body, and comprising:

an inner tube which runs the length of the system and has a distal endand a proximal end;

a wire coil disposed around at least a portion of the inner tube, havinga distal end, a proximal end, a distal region, an intermediate regionand a proximal region; and

an outer tube, having a distal end and a proximal end, disposed aroundat least a portion of the wire coil,

characterized by:

the wire coil having a closed-coil structure in the intermediate regionand an open-coil structure in at least one of the distal region and theproximal region; and by a flow path for liquid from the proximal to thedistal end of the system which includes a radially-extending portionthrough said open-coil structure.

According to another aspect of the present invention there is provided asystem for a medical apparatus comprising:

a tube member having a distal end and a proximal end; and

a sheath, having a distal end and a proximal end, disposed around atleast a portion of the inner tube, said sheath being retractable in aproximal direction relative to the tube to perform an actuating step atthe distal end of the system, by the application of an endwise tensilestress to the proximal end of the sheath,

characterized in that:

the tube member resists the associated radially- inward contraction ofthe sheath which arises from the applied tensile stress during saidactuating step.

According to various embodiments of medical surgical devices, there maybe provided any, some or all of the following features eitherindependently or in combination and according to any possiblearrangement.

The inner tube can be made of any suitable material, such as a metal orpolymeric material, having a distal end and a proximal end. The innertube may define a lumen therethrough, which may be a lumen suitable forthe insertion and retraction of a guide-wire. The inner tube is highlyflexible and capable of navigating the tortuous channels such as in thebiliary tree, arterial or venous system within the human body, and alsoprovides some resistance to compression.

The wire coil, preferably a reinforcing coil, is made from abiologically compatible material, preferably a metal such asnickel-titanium alloy or stainless steel. The coil is disposed aroundthe inner tube and provides torsional, radial and compressivereinforcement for that tube. At least one, and preferably both, of aproximal region and a distal region of the wire coil has an open-coilstructure, whilst an intermediate region of the wire coil has aclosed-coil structure. The wire coil is flexible and able to navigatethe above-mentioned tortuous passageways defined by inter-connectingblood vessels. The wire coil is elastically extendable and compressible,but will remain substantially uncompressed under the attendantcompressive forces normally encountered by such a member during theroutine operational procedures required to operate the medical surgicaldevice.

The outer tube is disposed about at least a portion of the inner tube.Preferably, the outer tube is a shrink-tube layer constraining the wirecoil. The outer tube can be of any suitable material, and is preferablymade from low friction PTFE. The outer tube is flexible and able tonavigate tortuous bodily lumens. The outer tube gives added stability,and therefore compressive strength, to the wire coil, and preventskinking of said coil during advancement through a tortuous passageway.

The wire coil in combination with the inner and outer tubes can define afluid flow path between the inner and outer tubes and along a helicalpath defined by the adjacent turns of the wire coil. The fluid flow pathmay alternatively comprise a helical or an annular path between the coiland the outer tube, or between the coil and the inner tube, for examplewhen the coil structure is such that substantially no fluid can passfrom inside the coil to outside the coil or from outside the coil toinside the coil between the adjacent turns of the closed-coil structure.

There may be provided an outer sheath disposed around the outer tube,that advantageously has little or even no radial gap between the sheathand the outer tube. The outer sheath is made from any suitable polymericmaterial, and is preferably made from a thermoplastic elastomer such asthe one sold by Du Pont under the trademark HYTREL. Although there ispreferably minimal gap between the sheath and the outer tube, there maybe provided a layer of lubricious material between the two, which may bea silicone coating on the outer surface of the outer tube. The sheathmay even be in a circumferentially pre-stressed condition if the outsidediameter of the outer tube is selected to be greater than the unstressedinner diameter of the sheath.

In a medical surgical device comprising an inner tube, wire coil, outertube and sheath, the inner tube and sheath preferably extend from aproximal end to a point distal of the distal ends of the wire coil andouter tube. There may be provided at the end of the medical surgicaldevice a tip. The tip may be formed as an atraumatic, tapered extensionof the distal end of the sheath or the tip may be provided at the distalend of the inner tube, optionally as a separate element attached to theinner tube. Such an attached tip may be made from any suitable softmaterial such as a polyurethane like Pellethane (a registered trademarkof Dow Chemical). The tip may be secured to the inner tube with asuitable adhesive, such as those sold by DYMAX Corporation.

In one preferred embodiment, the wire coil extends proximally of theouter tube by about 10 mm and extends distally of the outer tube byabout 10 mm, with the proximal region and distal region of the wire coilbeing within (or comprising) the portions of the coil length extendingbeyond the outer tube and just inside it, whilst the intermediate regionis the remaining, closed-coil, main part of the length of the wire coillying within the lumen of the outer tube.

There may be provided near the distal end of the medical surgical devicea pusher element disposed around the inner tube and adjacent the distalend of the wire coil. The pusher element can be attached to the wirecoil distal end, for example by glueing with DYMAX glue. Attachment isachieved preferably without glue. One way to do this is to have an endregion of the coil overlap radially outwardly over aproximally-extending annulus of the pusher and butting up against aproximal-facing shoulder of the pusher element.

The pusher element is advantageously made from PEEK. The pusher elementmay comprise a relatively long, cylindrical, proximal portion and ashorter, distal shoulder portion of larger diameter, with opposedshoulder surfaces facing proximally and distally. The pusher element issuitable for receiving on the distal-facing shoulder surface a proximalend face of an annular medical device such as a stent or stent graft,the proximal end of the medical device abutting the pusher element.

There may be provided a medical device at or near a distal end of themedical surgical apparatus. The medical device may be a self-expandingstent. The medical device can be disposed about a distal region of theinner tube which extends distally beyond the distal ends of the wirecoil and outer tube. The medical device is accommodated proximal of anytip at the distal end of the medical surgical device and distal of anypusher element at the distal end of the wire coil. The medical devicemay be held in position by a sheath extending over at least a portionof, and preferably the entire length of, the medical device. A medicaldevice which is a self-expanding stent or stent graft can be held withinthe sheath in a first, compressed state, to be released by retraction ofthe sheath in a proximal direction relative to the inner tube, wire coiland outer tube, the medical device being prevented from movingproximally by endwise compressive stress within the wire coil and anypusher element. When released, the medical device can expand, or beexpanded, to a second, non-compressed state.

There may be provided at the distal end of the sheath a radiopaquemarker and there may be provided at the distal end of the wire coil aradiopaque marker, such that release of the medical device is indicatedby a movement of the radiopaque marker on the sheath to a positionaligned with the radiopaque marker at the distal end of the wire coil,which indication can be viewed using visualizing means such asradioscopy or fluoroscopy. The radiopaque markers also serve to indicatethe correct positioning of the medical device within a blood vessel orother body lumen during insertion and advancement of the medicalsurgical apparatus.

There may be provided at the proximal end of the medical surgical deviceactuating means connected, directly or indirectly, to the proximal endsof any inner tube, wire coil, outer tube or sheath. The actuating meansis capable of holding the inner tube, wire coil and outer tube in place,and retracting the sheath in a proximal direction relative to the innertube, wire coil and outer tube. The actuating means may be connected tothe individual members directly, using means such as gluing, welding,etc. The inner tube and wire coil may be glued to a portion of theactuating means proximal of the portion of the actuating means to whichthe sheath is glued. Depending on the polymer present, the glue may beDYMAX glue.

The actuating means may include a Luer connection that communicates withthe inner tube, wire coil and outer tube, the Luer providing access toan internal lumen of the inner tube.

The sheath may be connected to a second Luer providing access to a fluidflow path between the inner tube, outer tube and adjacent turns of thewire coil.

The actuating means may further include a Tuohy Borst valve, preventingfluid escaping from the proximal end of the medical surgical device. Theactuating means may further comprise a swivel nut to lock the positionof the sheath relative to the outer tube during insertion.

The actuating means may also provide a safety clip to prevent accidentalretraction of the sheath during deployment.

In a medical surgical device comprising a wire coil between two tubularmembers, and where the wire coil has an open-coil structure at aproximal region and a distal region and a closed-coil structure in anintermediate region between the distal region and proximal region, aphenomenon is created whereby it is possible to introduce fluid into andthrough the gaps between the turns of the coil at a proximal region andthen with a pressure differential urge said fluid along a fluid flowpath defined between the adjacent turns of the coil and bounded outsideand inside by the outer tube and the inner tube respectively. In thisway, it is possible to advance the fluid along the helical path betweenthe turns of the coil, even in the closed-coil structure of theintermediate section. In the case where the outer tube is tight on theouter cylindrical surface of the coil, but the inner tube is less tighton the luminal cylindrical surface of the coil, fluid can flow within anannular gap between the inner tube and the turns of the coil. Theopen-coil structure at the distal end of the coil allows an increasedvolume flow of fluid to exit radially outwardly at the distal end andincreases the overall flow rate. In this way, no separate lumen isrequired for transferring the fluid, which may be a flushing fluid orvisualizing fluid, and therefore results in a reduced overall diameterof the device. Because the main supporting member is the wire coil, thedevice remains highly flexible for advancement through tortuouspassageways, yet is resistant to compressive forces associated with theretraction of an outer sheath. Therefore, the device can be advancedthrough narrower blood vessels whilst maintaining the required levels ofcontrollability and ensuring accurate deployment of any medical devicesto be delivered.

In one embodiment, an outer catheter sheath is disposed about the tubemember, and the sheath extends along substantially the entire length ofand distal of the tube member to enclose a compressed medical device,with the proximal end of the medical device abutting the distal end ofthe tube member, and the medical device having an equal or lesserdiameter to the tube member when compressed. In this embodiment, noseparate distal sheath is required. Because the proximal end of themedical device being inserted abuts the distal end of the inner tubemember and both are covered by the outer sheath, no separate actuatingmember is required within the medical surgical device for proximalretraction of the sheath at the distal end, since this is achieved at aproximal end of the medical surgical device, resulting in areduced-diameter delivery apparatus. Further, in this way, the diameterof the medical surgical device does not increase at the distal end wherethe medical device is located but remains constant along the entirelength of the medical surgical device.

Alternatively, for delivery of medical devices having an irreduciblediameter larger than what is needed for the shaft of the deliverysystem, the shaft may have a diameter substantially less than the distalend of the system where the medical device is housed. Where thepolymeric sheath. extends for substantially the entire length of themedical surgical device, and is disposed tightly around the tube member,and a tensile stress is applied to the outer sheath when the sheath isbeing retracted by actuating means at the proximal end, the outer sheathwould normally tend to contract radially inwards, as the endwise tensilestress increases. In preferred embodiments of the invention, thistendency is inhibited, because the tube member located closely insidethe outer sheath resists this radially inward motion. Hence, thetwo-dimensional stress matrix formed in the thin sheath is altered andresults in the sheath behaving, effectively, less elastically. Thismeans that the sheath will not stretch when the tensile force isapplied, and allows for accurate deployment of the medical devicecontained therein. Further, the material of the outer sheath is observedto be capable of bearing an increased endwise tensile stress before ityields. The effect can be used to enable the sheath to be made withoutany reinforcing wire or braided structure or with reduced wall thicknessand, consequently, of smaller outside diameter. This allows the designerto achieve a reduced overall diameter of the medical surgical device.

In a device as described above, it has been found that an inner tubemember of PTFE, and an outer sheath of HYTREL with a lubricious materialdisposed between the two, possibly silicone, results in a very smallcoefficient of friction between the two layers. Indeed, these twomaterials behave almost as if they repel one another, which assists inretraction of the sheath for deployment of a medical device.

Further advantage is provided because of the ease of manufacture andassembly of the surgical medical apparatus. This is due to the ease withwhich the selected materials can be manufactured, and also due to thesmall number of component parts required to build such a device. Thisought to reduce manufacturing time and costs.

It will be appreciated that the present invention is characterized bythe support which the inner tube gives to the outer tube when the outertube is in endwise tension when called upon to release from its distalend a medical implant such as a self-expanding stent. This supportmanifests itself in a smaller contraction of the diameter of the outertube during release of the implant than would be the case in the absenceof the inner tube. Accordingly, one way of discovering whether anyparticular co-axial delivery device utilises the teaching of the presentdisclosure is to examine the magnitude of diametral contraction, withand without the presence of the inner tube. If the presence of the innertube fails to reduce the amount of diametral contraction, this wouldsuggest that the support taught herein is not being given by the innertube to the outer tube. In addition, the shape of the stress/elongationplot for the outer sheath is significantly different, depending whetherthe outer sheath is receiving support from the inner tube assembly

Preferred embodiments of the invention will now be described, by way ofexample, and with reference to the accompanying drawings in which:

FIG. 1 is a longitudinal diametrical section through a stent deliverysystem according to a first embodiment of the invention;

FIG. 2 is a longitudinal diametrical section through a stent deliverycatheter according to a second embodiment of the invention; and

FIG. 3 is a longitudinal diametrical section through one possibleembodiment of a medical surgical device.

FIG. 1 shows a system having a proximal end and a distal end. At theproximal end there is an actuating means 150, 152, 250, 252 connected toan elongate tube member 100 within the lumen of a sheath 202. There is alubricious coating 132 applied to substantially the entire length of thetube member, which fills the annulus between the tube member and thesheath. The actuating means consists of a first connecting section 150joined to the tube member 100 and having a pair of finger loops 152, anda second connecting means 250 attached to the sheath 202 having secondmanual gripping means also in the form of finger loops 252. The systemhas a central bore 104 therethrough, suitable for the insertion andretraction of a guide wire. The tube member 100 may be a single tube ofpolymeric material, or it may have a composite structure. The outsidesurface of the tube member is made from PTFE. The sheath is made fromthe thermoplastic elastomer HYTREL. The lubricious coating is a thincoating of silicone.

In operation, the actuating means is held in the open position, withconnecting means 150 and 250 spaced apart, whilst the distal end of thesystem is inserted into and advanced along a body lumen. Once the distalend has reached the treatment site, the device is operated by retractingthe actuator 250 proximally, which moves the actuating means to a closedposition, with connecting means 150 and 250 adjacent one another.Closing the actuating means causes the sheath 202 to retract in aproximal direction relative to the tube member 100.

It is conceived that there may be located at the distal end of thesystem any one of a range of mechanisms operated by the retraction ofsaid sheath, although the system is most immediately contemplated as foruse as a catheter to deliver a stent or similar endoluminal medicalsurgical devices.

Because the sheath extends for substantially the entire length of thetube member, there will be a tensile force within the sheath duringretraction, due to friction between the sheath and the distal device andthe inner tube, and a resisting compressive force within the tubemember. The gap between the tube member and sheath is sufficiently smallto reduce radially inward contraction of the sheath under tensilestresses, thereby helping to prevent the sheath from stretchinglengthwise. Further, the friction forces between the sheath and the tubemember are kept small by judicious selection of the involved materials,which helps to prevent the compressive stress in the tube member fromdeforming it and displacing the distal tip proximally, which isparticularly undesirable in an endoluminal device delivery system.

FIG. 2 shows a cross-sectional view through a member forming part of amedical surgical apparatus. The member comprises a flexible inner tube102, which is made from a polymeric material. At a distal end of theinner tube there is a tip 106, which is made from Pellethane. The innertube and distal tip have an axial bore 104 running therethrough, whichforms a guide wire lumen. Disposed around the inner tube, from theproximal end to a point near the distal end of the inner tube, is a wirecoil 110. The wire coil is wound around the inner tube and has a distalregion 114, a proximal region 112 and an intermediate region between thetwo. As illustrated, the wire coil has an open-coil structure in thedistal region and proximal region, and a closed-coil structurethroughout the intermediate region and in the short zones at the veryproximal and distal ends of the coil. The wire-coil is made fromstainless steel, providing the necessary flexibility and compressionresistance.

At the distal end of the wire coil is a pusher element 118. The pusherelement consists of a wider, proximal flange-like portion 120 and anarrower distal shoulder portion 122 which receives radially about itthe very distal end of the wire coil 110. The pusher element is madefrom PEEK, and is either attached to the wire coil using DYMAX glue orthe distal end of the coil is tight enough around the cylinder 122 toneed no glue. There is formed between the pusher element 118 and thedistal tip 106 a region suitable for receiving a medical device forinsertion into a body lumen. In particular, this medical devicereceiving area is capable of holding a self-expanding stent, with thedistal end of the stent abutting the tip 106 and the proximal end of thestent disposed around the shoulder 122 of the pusher element andabutting the flange portion 120. The region may therefore be termed a“stent bed”.

Disposed around the intermediate region of the wire coil 110 is an outertube 124. This tube is a shrink-tube made of PTFE and fits tightlyaround the turns of the coil. The outer tube acts to constrain the wirecoil in the desired shape and inhibits the coil from kinking duringadvancement of the member through a tortuous passageway, such as a bodylumen. Because the outer tube is tight around the coil it inhibits theadjacent turns of the coil from overlapping one another. In this way,the wire coil structure provides good resistance to compressive forcesoccurring within the member. Prior to insertion of the member into abody lumen, flushing fluid can be passed through the fluid flow pathdefined between the inner tube, outer tube and adjacent turns of thewire coil. During a surgical procedure visualizing fluid may betransported from a proximal end of the member to a distal end of themember along the same fluid flow path. A gap between the coil and theinner tube 102 is the main fluid flow path in the intermediate region ofthe wire coil.

FIG. 3 shows a medical surgical device for delivering a self-expandingstent. The device comprises a proximal end and a distal end with anelongate, tubular intermediate region. The apparatus comprises an innercatheter 100 and an outer sheath member 200.

The inner catheter comprises an inner tube 102 made from polymericmaterial having a lumen 104 therethrough which is suitable to be used asa guide wire lumen. At the distal end of the inner tube there is a tip106, made from Pellethane, glued to the distal end of the inner tubewith DYMAX glue. Disposed about the inner tube, at a proximal region ofthe inner tube, is a rigid, stainless steel push rod 108. A stainlesssteel wire coil 110 abuts the distal end of the push rod and is disposedaround substantially the remaining length of the inner tube, except fora small distal region of the inner tube proximal of the Pellethane tip.The wire coil has a proximal region 112 and a distal region 114 havingopen-coil structures and an intermediate region 116 having a closed-coilstructure. Abutting the distal end of the wire coil and disposed aroundthe inner tube is a pusher element 118 made from PEEK consisting aproximal flange-like portion 120 and a distal shoulder portion 122. Thepusher element is joined to the wire coil (not shown in FIG. 3) asexplained above by reference to FIG. 2. The wire coil is joined at theproximal end to the distal end of the push rod, either in like manner orin any other way chosen by a person skilled in designing stent deliverysystems. Disposed around the intermediate region of the wire coil is aPTFE shrink-tube 124 constraining the wire coil and preventing the coilfrom kinking during advancement through a tortuous passageway. Thishelps to increase the compression resistance of the wire coil, bypreventing the adjacent turns of the coil from overlapping, withoutsubstantially reducing the flexibility of the wire coil. At the proximalend of the inner catheter 100 is a Luer 126 and a swivel nut 128connected to a first part 150 of a proximal actuating means. Luer 126provides access to the guide wire lumen 104.

The outer tubular member comprises a sheath 202 extending from near theproximal end of the inner tubular member to the distal end of the innertube and abutting the Pellethane tip. The sheath is disposed tightlyabout the PTFE shrink-tube 124 of the inner catheter 100, leavingsubstantially no gap between the two, the existing gap being filled by alubricious material 132, which is a silicone coating on the outersurface of the PTFE shrink-tube. The sheath is slideable in a proximaldirection over the inner catheter and is made from the thermoplasticelastomer HYTREL.

At the distal end of the sheath, on an inner surface, there is aradiopaque marker band 204 for determining the location of the distalend of the surgical apparatus during insertion through a body lumenusing visualizing means such as radioscopy. The pusher 120 functions asa further radiopaque marker band located at the proximal end of thestent bed. When the sheath is retracted, complete retraction isobservable by the radiopaque marker band 204 on the distal end of thesheath 202 aligning with the radiopaque marker band 130 at the distalend of the wire coil 110.

At the proximal end of the outer tubular member there is a second part250 of the proximal actuating means, also comprising a swivel nut 206for locking the position of the outer sheath member 200 relative to theinner coil member 100, to prevent unwanted retraction of the sheathduring insertion of the medical surgical device. There is a second Luer208 connected with the second part 250 of the actuating means to provideaccess to the fluid flow path 134 defined between the inner tube 102,outer tube 124 and adjacent turns of the wire coil 110. Access into theflow path is achieved by fluid passing into the open-coil structure ofthe wire coil at the proximal end, via the second Luer and then into thegaps between the coils between the inner tube and outer tube. There isfurther provided a Tuohy Borst valve 210 to prevent fluid escapingproximally from the sheath member. Disposed around a proximal region ofthe sheath is a rigid, tubular support member 212.

A self-expanding stent 300 is contained within the apparatus at a distalregion in the stent bed between the Pellethane tip 106 and PEEK pusherelement 118. The proximal end of the self-expanding stent 300 abuts theflange portion 120. This prevents the stent from moving proximallyrelative to the inner tube 102 during retraction of the sheath 202. Thesheath is disposed over the entire length of the self-expanding stentand maintains the stent in a first, radially compressed state. When thesheath is retracted, the stent begins to expand and is held in thecorrect deployment position by forces from the sheath and pusherelement. The operation of the medical surgical apparatus will now bedescribed. Prior to the insertion into a body lumen, any air within thedevice is removed by passing flushing fluid from the proximal end to thedistal end of the device through the fluid flow path 132 via the secondLuer 208, and through the central lumen 104 via the first Luer 126.Swivel nut 206 is tightened to lock the position of the outer sheathmember 200 relative to the inner coil member 100. The tip 106 of themedical surgical apparatus is then inserted into a body lumen. Itfollows a path along a previously inserted guide wire which runs throughthe inner lumen 104. The apparatus is advanced until the distal endreaches the treatment site.

The position and orientation of the medical surgical apparatus withinthe body lumen can be visualized using a radioscope to monitor pulses ofcontrast fluid injected into the body lumen through the internal lumen134 of the medical surgical device. Radiopaque markers 204 and 120 canthen be used to correctly position the stent across the treatment zone.

Once the stent is correctly positioned, the swivel nut 206 is released,and the sheath 202 can be retracted by moving the second part of theactuating means 250 proximally towards the first part of the actuatingmeans 150. As the sheath is retracted, the self-expanding stent 300 isreleased, and expands to engage and hold apart the inner wall of thebody lumen. Once the radiopacue marker bands 204 and 120 have beenobserved to be aligned, the stent is fully deployed and the medicalsurgical apparatus can be retracted, the Pellethane tip able to returnproximally through the inner lumen of the expanded stent.

The illustrated embodiments are Examples within the scope of the clamsthat follow. The invention is applicable to a range of implant deliverysystems beyond those for self- expanding stents. It is applicable toboth over the wire and rapid exchange systems, as well as to systemswhich lack any guidewire lumen at all. Evidently the skilled reader willbring his/her specialist background knowledge to bear, when extractinguseful teaching from the above description.

1. A catheter for a medical apparatus comprising: an inner catheter (100) having a distal end and a proximal end; and a sheath (202) of polymer material, having a distal end and a proximal end, disposed around at least a portion of the inner catheter, said sheath being retractable in a proximal direction relative to the inner catheter to perform an actuating step at the distal end of the system, by the application of an endwise tensile stress to the proximal end of the sheath, characterized in that: the inner catheter resists the associated radially- inward contraction of the sheath which arises from the applied tensile stress during said actuating step.
 2. Catheter as claimed in claim 1 and including a lubricious coating (132) on the outer surface of the inner catheter.
 3. Catheter as claimed in claim 1 or 2, with a lubricious fluid (132) in the annulus between the sheath and the inner catheter.
 4. Catheter as claimed in any one of the preceding claims, wherein the sheath is made from a thermoplastic elastomeric material.
 5. Catheter of any preceding claim, wherein the inner catheter comprises: a wire coil (110) having a lumen, a distal end, a proximal end, a distal region (114), an intermediate region and a proximal region (112); and an outer tube (124) disposed around at least a portion of the wire coil, having a distal end and a proximal end.
 6. Catheter of claim 5, in which the inner catheter includes an inner tube (102) radially within the wire coil lumen
 7. Catheter of claim 6, wherein the distal end of the inner tube extends to a point distal of the distal end of the wire coil.
 8. Catheter as claimed in claim 6 or 7, wherein the inner tube defines an inner lumen (104) for the insertion and retraction of a guide-wire therethrough.
 9. Catheter as claimed in any one of claims 5 to 8, wherein the wire coil has a closed-coil structure in the intermediate region and an open-coil structure in at least one of the distal region and the proximal region.
 10. Catheter of claim 9, wherein the wire coil has an open-coil structure in both the distal region and the proximal region.
 11. Catheter as claimed in any one of claims 5 to 10, in which the wire coil defines a liquid flow path from the proximal end to the distal end of the system which includes a radially-extending portion through said open coil structure, for transferring liquid from the proximal end to the distal end of the coil.
 12. Catheter as claimed in claim 1 1, wherein the liquid flow path is an annular flow path bounded inside by the inner tube and outside by the wire coil, for transferring liquid from the proximal end to the distal end of the coil.
 13. Catheter as claimed in any one of claims 5 to 12 wherein the outer tube is a shrink-tube constraining the wire coil.
 14. Catheter as claimed in claim 13, wherein the outer tube is made from PTFE.
 15. Catheter as claimed in any one of claims 5 to 14, wherein the inner tube defines a medical-device-receiving annulus around a distal portion of the inner tube, said distal region being distal of the distal end of the wire coil and proximal of the distal end of the inner tube.
 16. Catheter of any preceding claim, and having an atraumatic tapered tip (106) at the distal end of said system.
 17. Catheter of claim 16, wherein the tip is formed as part of the sheath.
 18. Catheter of claim 16, wherein the tip is attached to the inner catheter.
 19. Catheter of claim 18, wherein the tip is made from a polyurethane.
 20. A medical-device-delivery apparatus, having a distal end and a proximal end, comprising: a catheter as claimed in any one of claims 5 to 19; actuating means (150, 250) connected to the inner catheter and to the sheath for retracting the sheath in a proximal direction relative to the inner catheter.
 21. The medical-device-delivery apparatus of claim 20, and carrying a medical device (300) for deployment in a bodily lumen, the medical device being maintained in position between the sheath and the inner catheter, and the medical device being releasable by a retraction of the sheath in a proximal direction relative to the inner catheter.
 22. The medical-device-delivery apparatus of claim 21, wherein: the medical device is held within the lumen of the outer sheath at a location distal of the distal end of the wire coil; the medical device is maintained radially compressed in a first state by the sheath being disposed around at least a portion of the medical device; during retraction of the sheath, the medical device is prevented by the wire coil from moving with the sheath in a proximal direction; and when the sheath is retracted in a proximal direction relative to the inner catheter, the medical device is released for expansion to a radially less compressed state.
 23. The medical-device-delivery apparatus of claim 21 or 22, wherein the medical device is a self-expanding stent. 